Experimental and chemical kinetics analysis on the combustion behaviors and flame instabilities of diluted methane/air mixtures.

• The concavity of spherical flame formed by the synergy of buoyancy vs. pure hydrodynamics. • A linearity of maximum pressure vs. LBV was established by incorporating the flame thickness. • Inhibition of methane combustion by the thermal effect of diluents is stronger than chemistry. • CO 2 /H 2 O chemical effect can weaken the thermal-diffuion and Darrieus-Landau instabilities. The evolutions of spherical flame of methane/air/diluent mixtures in a cylindrical chamber were observed by setting different equivalence ratios ϕ and diluent volume fraction V d i l u t i o n s , and the relationship of combustion pressure vs. chemical parameters was improved. Moreover, the influences of ϕ on the methane combustion characteristics and flame instability were explored by chemical pathways. Based on the results obtained, the spherical flame exhibits asymmetric propagation and generates a concavity in its lower flame front that closely resembles the "Tulip flame" at higher V d i l u t i o n s. The dominant mechanism responsible for this phenomenon is the pure hydrodynamics influenced by stronger buoyancy effects (i.e., F r o u d e n u m b e r ≤ 0.016). Moreover, a linear correlation of laminar burning velocity (LBV) with the maximum combustion pressure p m a x modified by flame thickness δ , which explains that p m a x is primarily determined by the heating rate and combustion velocity. Results from the sensitivity analysis of LBV reveal that simplifying the mechanisms for CH 4 -rich and CH 4 -lean mixtures must be differentiated to reduce the calculated quantity and enhance calculated accuracy. Furthermore, CO 2 /H 2 O chemical effects only contribute to the inhibition of LBV under the extremely CH 4 -lean conditions due to the influences of ϕ on CO 2 /H 2 O chemical pathways. In addition, the thermal diffusion and flame thickness are sensitive to the chemical pathways associated with OH and H radicals, respectively. Therefore, CO 2 /H 2 O chemical effects both weaken the influences of Thermal-diffusion and Darrieus-Landau instabilities on methane flames by affecting the productions of H and OH radicals, and H 2 O chemical effect can relieve flame concavity due to the improvement of the sensitivity of flame front on stretch effect. [ABSTRACT FROM AUTHOR]